Field of the Invention
The present invention relates to a single crystal ferrite composition suitable for magnetic heads used in record and reproduction of images in video cassette recorder (VCR) systems, and more particularly to an Mn-Zn-based single crystal ferrite composition capable of achieving a high picture quality and a high resolution of images in a high frequency region of 7 to 8 MHz.
Description of the Prior Art
Generally, Mn-Zn-based single ferrite materials exhibit a high specific resistivity, a high initial magnetic permeability, and a high wear resistance. Moreover, they have an advantage of a high saturated magnetic flux density. By virtue of such advantageous properties, Mn-Zn-based single ferrite materials have been mainly used for fabrication of magnetic heads used in record and reproduction of images in VCR systems.
Where magnetic heads made of such conventional Mn-Zn-based single crystal ferrite materials are used in lower-grade VCR systems in which a high picture quality and a high resolution are not required, they could perform their functions sufficiently in a frequency range of 4 to 5 MHz.
Recently, however, demand for higher picture quality and higher resolution has been increased. The existing magnetic heads equipped in the lower-grade VCR systems could not satisfy such a demand.
For this reason, there have recently been made developments of new image-recording and reproducing magnetic heads exhibiting a high initial magnetic permeability in the frequency range of 7 to 8 MHz and a reduced rubbing noise so as to satisfy the demand for higher picture quality and higher resolution.
Now, manufacture of an existing Mn-Zn-based single crystal material and fabrication of an image-recording and reproducing magnetic head using the Mn-Zn-based single crystal material will be described.
At a first step S.sub.1, ferric oxide (Fe.sub.2 O.sub.3), manganese oxide (MnO) and zinc oxide (ZnO) are prepared in the form of powder and then mixed together to obtain a basic composition of Mn-Zn-based single crystal ferrite.
In the basic composition, Fe.sub.2 O.sub.3, MnO and ZnO have contents of 51 to 54 mole %, 27 to 33 mole %, and 16 to 20 mole %, respectively.
Upon mixing, other materials than Fe.sub.2 O.sub.3, MnO and ZnO are not added optionally.
The following description will be made in conjunction with a composition essentially consisting of 53 mole % Fe.sub.2 O.sub.3, 28 mole % MnO and 19 mole % ZnO, for the simplicity of the description.
At a second step S.sub.2, the composition is subjected to a homogeneous blending using a wet ball milling process. By the homogeneous blending, Fe.sub.2 O.sub.3, MnO and ZnO of the composition are in the form of slurry.
The resultant slurry is then dried using a spray dry process at a third step S.sub.3, thereby producing powder.
At a fourth step S.sub.4, the powder is sintered at a temperature of 1,300.degree. C. for 4 hours.
The resultant sintered body is then grown on a &lt;100&gt; oriented single crystal seed using the vertical Bridgeman process at a fifth step S.sub.5. By this growth, an Mn-Zn-based single crystal ferrite ingot is obtained.
At a sixth step S.sub.6, the leading end of the ingot is cut. Thereafter, the cut surface of the ingot is subjected to an etching using an HCl solution. The orientation of the etched surface of the ingot is then determined by use of a measuring device using He laser.
At a seventh step S.sub.7, the ingot subjected to the orientation measurement is cut to obtain blocks having a thickness of 1.6 cm. Then, each block is cut to obtain wafers having a thickness of 1 mm. Each wafer obtained is subjected to the measurement of specific resistivity using a 4-point probe measuring process.
From the wafers, toroidal samples each having an inner diameter of 5 mm, an outer diameter of 8 mm and a thickness of 0.5 mm are formed at an eighth step S.sub.8. The toroidal samples are subjected to measurements of the saturated magnetic flux density B.sub.10, the initial magnetic permeability .mu.i and the temperature dependency to the magnetic permeability .mu.. The results will be described in conjunction with Tables 1 and 2.
As shown in Table 1, the saturated magnetic flux density B.sub.10 of 4,200 to 5,000G is exhibited in the toroidal sample obtained in the case where the Mn-Zn-based single crystal ferrite contains a basic composition essentially consisting of 51 to 54 mole % Fe.sub.2 O.sub.3, 27 to 33 mole % MnO and 16 to 20 mole % ZnO without any additional elements. In this case, the toroidal sample also exhibits the initial magnetic permeability .mu.i of 200 to 330 at the frequency of 7 MHz. On the other hand, the temperature T.sub..mu.2p corresponding to a secondary peak of magnetic permeability .mu. is ranged from 0.degree. to 50.degree. C. at the frequency of 0.5 MHz.
In particular, where the Mn-Zn-based single crystal ferrite contains a basic composition essentially consisting of 53 mole % Fe.sub.2 O.sub.3, 28 mole % MnO and 19 mole % ZnO without any additional elements, the toroidal sample obtained from this single crystal ferrite exhibits the saturated magnetic flux density B.sub.10 of 4,900G and the initial magnetic permeability .mu.i of 300 at the frequency of 7 MHz, as shown in Table 2. In this case, the temperature T.sub..mu.2p corresponding to the secondary peak of magnetic permeability .mu. is 20.degree. C. at the frequency of 0.5 MHz.
Using the Mn-Zn-based single crystal ferrite, an image-recording and reproducing magnetic head having a construction shown in FIG. 5 is fabricated at a ninth step S.sub.9. The magnetic head has a gap G of 0.37 mm and a track pitch of 31 .mu.m. In FIG. 5, the reference numeral 211 denotes a contact surface of the magnetic head to be in contact with a magnetic tape.
Finally, the magnetic head is subjected to measurements of the reproduction output and the rubbing noise at a tenth step S.sub.10. The results are shown in Tables 1 and 2.
As shown in Table 1, the reproduction output of 17 to 220 .mu.V and the rubbing noise of 5 to 8 dB are measured in the magnetic head obtained in the case where the Mn-Zn-based single crystal ferrite contains a basic composition essentially consisting of 51 to 54 mole % Fe.sub.2 O.sub.3, 27 to 33 mole % MnO and 16 to 20 mole % ZnO without any additional elements. In particular, where the Mn-Zn-based single crystal ferrite contains a basic composition essentially consisting of 53 mole % Fe.sub.2 O.sub.3, 28 mole % MnO and 19 mole % ZnO without any additional elements, the resultant magnetic head generates the reproduction output of 190 .mu.V and the rubbing noise of 6 dB, as shown in Table 2.
The rubbing noise generated from the magnetic head, namely, the value of 5 to 8 dB shown in Table 1 is considered as being relatively high. Such a high rubbing noise is caused by the fact that the magnetic domain of a magnetic tape being in contact with the magnetic head is easily varied due to its unstably fixed condition.
Since the conventional magnetic head exhibits a relatively low initial magnetic permeability and a relatively high rubbing noise at the frequency of 7 to 8 MHz, it is unsuitable as a magnetic head for high-grade 8 mm camcorders and super VHS VCRs widely used at present.